It is is known that the corrosion resistance of steel strips can be improved by depositing thereon protective layers of coatings of organic or metallic materials. Generally, such coatings are deposited continuously with the assistance of appropriate installations.
The metallic coatings can be deposited on the strips to be protected either by using a dip or electrolytic coating method.
In the dip coating method, the strip is dipped into a bath formed of the melted coating metal. Taking into account the melting points of the metals likely to form an anti-corrosion protective coating, the number of metals which can thus be used is rather limited. They are substantially: tin, lead, zinc, aluminum and all their alloys.
When practicing the electrolytic depositing method of the protective coating, coating metals can be used which have a higher melting point, and which afford anti-corrosion protection which is more efficient than that provided by the hereabove cited coating metals. Among the metals which can be used for electrolytic deposition, nickel or chrome can be named in particular. In addition, this method allows forming coating layers of several superimposed components, or intimately mixed, inside the layers.
It is understandable that the electrolytic deposition method provides a greater flexibility in the formation of the metallic coating layers, and protective coatings of higher corrosion resistance than that obtained by the dip coating method. However, the electrolytic depositing method exhibits the disadvantage that it cannot be associated, directly and in the same installation, with a strip annealing treatment, which is possible in the dip coating metallization installations. It follows from the foregoing that there is presently a need for systems associating a dip coating technology with means allowing forming coating layers containing elements having a high melting point and providing therefore an increased corrosion resistance. Consequently, the present invention provides a method which makes it possible to provide such composite coating layers in a metallization dip coating installation.